1. Field of the Invention
The present invention relates to an improved injection type blow molding machine, particularly to an an oval type die of the machine whereby a plasticizing material is injected through the die to provide a parison having an elliptical annular profile in a horizontal cross-sectional view. The parison is blow molded to produce a hollow double wall type plastic article having a basically concave, convex or bowl shape or a panel shape with a hollow double wall.
2. Description of the Related Art
A blow molding process using an extruding machine or an injection machine with a die having a circle die lip through which a parison having a circular profile in a vertical cross-sectional view is well known as a method of producing a hollow plastic article with a solid wall.
Recently, a modified blow molding process of producing a hollow plastic article with a hollow double wall, using an injection machine or an extruder with a an oval type die has been developed.
A conventional oval type die is designed so as to provide a parison having an elliptical annular profile in a horizontal cross-sectional view, rather than the circular profile used for the solid wall type hollow article. In this connection, the oval type die has a die lip or a die outlet at a lower end thereof having a four sided elliptical annular profile, which is rotated symmetrically about a vertical axis of the die. The profile has one pair of opposite sides which are linear and in parallel with each other, with a narrower width therebetween, and another pair of opposite sides which are convex, preferably semi-circular, with a longer width therebetween at the tops thereof. The oval type die is provided with a melt passage therein for communicating with a machine body and the die lip, and this passage has a profile, at the periphery thereof in a horizontal cross-sectional view, similar to that of the die lip.
It is known that the oval type die is defective when used with an injection machine, in that it is apt to provide a parison having an elliptical annular profile with a wall thickness which locally varies. This local thickness variation occurs due to a local deformation of the die, which leads to a local deformation of the die lip due to a high pressure, e.g., 500 kg/cm.sup.2 or more, of a plasticized material injected into the die. A parison having an uniform thickness produces a good blow molded article, whereas a parison having a locally varied thickness produces a defective article, in that it is locally weakened and/or is deformed relative to a profile of a mold cavity.
The deformation of the die lip is apt to occur at the linear sides of the die lip because the linear sides are bulged outward by the high pressure of the material at the centers thereof, and thus become, convex. Due to the above deformation, a die lip width of one of the linear sides is often larger than that of the other linear side.
Despite the above-mentioned problems, the injection type blow molding with the oval type die is critical when wishing to produce hollow double wall type articles having superior physical or mechanical properties such as a strength, a surface hardness, a thermal resistance, a weather resistance or the like, thereof. This is because such articles are made of an engineering plastic, as a plastic material, which has a characteristic such that a melt thereof is solidified at a high speed or velocity, compared with a normal plastic used in a extrusion type blow molding. In this regard, an injection machine must be used, to thus overcome the high speed solidifying characteristic of the engineering plastic.
It is known that there is a simple method of preventing the deformation of the die lip of the oval type die for use in the injection type blow molding, wherein the die per se is reinforced as a whole by an increase of a horizontal width thereof Nevertheless, this method is defective in that the reinforced die is heavier, and accordingly, the injection machine must be reinforced to carry the heavy die at a forward end thereof, which increases costs, and further, the heavy die makes handling difficult. Still further, the reinforced die is defective in that the thermal capacity thereof is accordingly increased, with the result that an increased thermal energy must be used to keep it at a desired high temperature. This increases the heating costs incurred for the injection blow molding.